In recent years, for HSDPA (High Speed Downlink Packet Access) which transmits packet data at high speed on a downlink, applying MIMO (Multi Input Multi Output) communication wherein a plurality of transmit antennas transmit respectively different packets of data simultaneously using the same frequency and the same code has been examined in order to further increase transmission speed. When performing the MIMO communication, signals transmitted from the plurality of transmit antennas are received by a mobile station apparatus provided with the same number of, or more, receive antennas as the transmit antennas of the base station apparatus, and a matrix operation is performed on the signals received by the respective receive antennas to separate into packets of data transmitted by the respective plurality of transmit antennas, which are then demodulated and decoded.
In HSDPA having the above MIMO communication applied thereto, when transmitting packets of data to a plurality of mobile station apparatuses, the base station apparatus may transmit packets of data from the plurality of transmit antennas respectively to the different mobile station apparatuses simultaneously. For example, a base station apparatus provided with two transmit antennas may assign one transmit antenna to a mobile station apparatus and the other transmit antenna to another mobile station apparatus, and transmit packets of data simultaneously.
In the case where a base station apparatus assigns a plurality of transmit antennas respectively to different mobile station apparatuses and transmits packets of data simultaneously as described above, if every mobile station apparatus receiving the packets of data has the same number of, or more, receive antennas as the transmit antennas of the base station apparatus, each mobile station apparatus can correctly separate the packets of data transmitted from the respective transmit antennas and demodulate and decode the packet of data addressed thereto.
However, if the number of receive antennas of a mobile station apparatus is less than the number of the transmit antennas of the base station apparatus, the mobile station apparatus cannot correctly separate the packets of data transmitted from the respective transmit antennas, so that packets of data addressed to other apparatuses become an interference component, thus causing the problem that receive quality degrades.